The low velocity impact response of a range of foam-based sandwich structures has been investigated using an instrumented falling-weight impact tower. Initially, the rate-sensitivity of the skin and core materials was investigated through a series of flexure and indentation tests. Here, it was shown that the flexural modulus of the skins and all 11 foam materials did not exhibit any sensitivity to crosshead displacement rate over the conditions studied here. In addition, it was shown that the indentation response of the sandwich structures could be modelled using a simple indentation law, the parameters of which did not exhibit any sensitivity to loading rate. Low velocity impact tests on the sandwich structures resulted in a number of different failure modes. Here, shear fracture was found to occur in the PVC/PUR systems based on brittle core materials. In contrast, buckling failures in the uppermost composite skin were observed in the intermediate modulus systems, whereas initial damage in the higher modulus PVC/PUR systems took the form of delamination within the top surface skin. It has been shown that a simple energy-balance model based on the dissipation of energy during the impact event can be used to successfully model the elastic response of foam-based sandwich structures. The energy-balance model is particularly useful since it can be used to establish the partition of energy during the impact process.

The low velocity impact response of foam-based sandwich structures

The low velocity impact response of two aluminium honeycomb sandwich structures has been investigated by conducting drop-weight impact tests using an instrumented falling-weight impact tower. Initially, the rate-sensitivity of the glass fibre reinforced/epoxy skins and aluminium core was investigated through a series of flexure, shear and indentation tests. Here, it was found that the flexural modulus of the composite skins and the shear modulus of the aluminium honeycomb core did not exhibit any strain-rate sensitivity over the conditions investigated here. In addition, it was found that the indentation characteristics of this lightweight sandwich structure can be analysed using a Meyer indentation law, the parameters of which did not exhibit any sensitivity to crosshead displacement rate. The impact response of the aluminium honeycomb sandwich structures was modelled using a simple energy-balance model which accounts for energy absorption in bending, shear and contact effects. Agreement between the energy-balance model and the experimental data was found to be good, particularly at low energies where damage was localised to the core material immediate to the point of impact. The energy balance was also used to identify energy partitioning during the impact event. Here, it was shown that the partition of the incident energy depends strongly on the geometry of the impacting projectile.

The low velocity impact response of an aluminium honeycomb sandwich structure

Impact damage can degrade the flexural strength of composite sandwich structures by over 50% due to a loss of skin support inducing localized skin buckling. Various self-healing methodologies have been applied to laminated composites but the concept of delivering a healing agent from a remote reservoir to a region of damage via a vascular network offers the potential for a robust and replenishable system housed in the core of a sandwich structure. In this pilot study a vascular sandwich structure that appears as a conventional sandwich composite has been developed and tested. The network has been shown to have negligible influence on the innate static mechanical properties of the host panel. Infiltration of the vascular network with a pre-mixed epoxy resin system after impact damage demonstrated a complete recovery of flexural failure mode and load. Infiltration with the same resin system from separate unmixed networks, where self-healing is initiated autonomously via mixing within the damage, has also been shown to fully recover undamaged failure load when both networks are successfully breached.

https://iopscience.iop.org/article/10.1088/0964-1726/16/4/031/pdf

Self-healing composite sandwich structures

This contribution is a review of past and current research progress on dynamic response of composites sandwich structures subjected to low-velocity impact. To obtain a comprehensive overview of the current state-of-the-art published works, impact responses on sandwich structures are broadly classified into two main groups, high-velocity and low-velocity impacts, with the focus on low-velocity impact. A summary of some of the commonly used theoretical solutions for low-velocity impact is provided and described in some details. The response of low-velocity impact has been determined based on mass ratio, which is defined as the ratio of the impact mass to effective mass of the structural component. As a result, the response under low-velocity impact was further subdivided into three possible categories, namely, large, small, and medium mass impacts. For each category of impact, currently available suitable solutions are discussed in details. To classify the response of low-velocity impact and also to explore...

A review of low-velocity impact on sandwich structures

The response of honeycomb sandwich panels under low-velocity impact loading

Because of high stiffness and strength to weight ratios, composite sandwiches are being used increasingly, especially in aerospace applications. The main drawback of sandwich components is their relatively low resistance to impact damage and the extent to which their strength is reduced under compressive loading after impact. As part of a wider study aimed at increasing the impact resistance of sandwich panels, a continuum damage model is proposed. The model developed describes the compressive behaviour of honeycombs made from materials that are prone to elastic buckling. The material behaviour in compression is described by a combination of three distinct constitutive models, namely elastic, elastic continuum damage and an inelastic strain accumulation model. This has been interfaced with a commercial finite element package to model soft body impacts onto a minimum gauge honeycomb core sandwich. Results from analysis are compared to experimental data and the correlation is found to be very good.

Modelling of Crushing Due to Impact in Honeycomb Sandwiches

The assessment of damage due to impacts in composite sandwich panels and the component life reductions associated with such damage is becoming increasingly important within the aerospace industry. This study analyses minimum gauge, non-metallic honeycomb wing panels subject to impact damage. In all instances the damage is caused by ‘soft body’ impactors travelling at elevated velocities to simulate bird strike and other soft debris. The damage formed during these impacts is shown to be large in plan area but shallow and primarily causes crushing of the core in a thin layer. Loading of the panels after impact has been performed to determine the reduction in load carrying capacity and associated failure mechanisms. Damage due to soft body impact is shown to be very different to the classical rigid body impact upon which current repair schemes are based. As a result in many applications the use of current repair schemes may be inappropriate. The viability of new repair techniques is discussed with particular attention to the prevention of failure mechanisms due to loading after impact.

R. R. Aitken

Papers

Modelling of Crushing Due to Impact in Honeycomb Sandwiches

Life extension of impact damaged honeycomb sandwich panels